1. Field of the Invention
The present invention relates to an accumulating multiplication circuit, and more specifically to an accumulating multiplication circuit having a function of executing a double-precision multiplication at a high speed.
2. Description of Related Art
A conventional accumulating multiplication circuit is basically constituted of a multiplication part and an arithmetical operation part, and operates as follows: The multiplication part receives two items of data of for example "n" bits, and outputs data of "2n" bits. The arithmetical operation part receives the data of "2n" bits outputted by the multiplication part and another data of "2n" bits, and outputs data of "2n" bits.
By using this conventional accumulating multiplication circuit, a multiplication of "2n" bits.times."2n" bits expressed in the 2'complement, that is, a double-precision multiplication will be executed as follows:
Here, supposing that a multiplicand and a multiplier are A and B, respectively, in the case of executing the multiplication of two numbers of "2n" bits, expressed in the 2'complement, the multiplicand and the multiplier can be expressed in the following formulas: ##EQU1##
Supposing that a product of A and B is P, P is shown by the following formula: ##EQU2##
Here, supposing that Q, R, S and T are expressed by the formulas (6) to (9), respectively, P can be shown by the following formula (10): ##EQU3##
Thus, the double-precision multiplication of "2n" bits can be executed by executing the following four kinds of single-precision multiplication and also executing the shift and addition for weighting of 2.sup.-2n or 2.sup.-n in accordance with the formula (10).
Q.multidot.R: 2'complement.times.2'complement PA1 Q.multidot.T: 2'complement.times.Absolute value PA1 S.multidot.R: Absolute value.times.2'complement PA1 S.multidot.T: Absolute value.times.Absolute value
In this case, the total number of the operations is six in the whole, that is, one accumulating multiplication of a 2'complement.times.a 2'complement, two accumulating multiplications of a 2'complement.times.an absolute value, one multiplication of an absolute value.times.an absolute value and two n-bit shifts. Namely, the required operation time is six times longer than that of the single-precision operation.
As mentioned above, in the case of executing a double-precision operation to obtain the operation results of "2n" bits, the conventional accumulating multiplication circuit as mentioned above requires the operation time six times as long as that of the single-precision multiplication. Further, it requires two times of saving of data carried over by the shifting in order to obtain the operation result of "4n" bits.